1. Field of the Invention
The present invention relates to a coaxial dielectric resonator used for a high-frequency filter in a communication apparatus such as a mobile communication apparatus and a personal handy phone system (PHS), and more particularly, to a small size coaxial dielectric resonator and a method for manufacturing the same.
2. Related Art of the Invention
In recent years, with the development of the mobile communication and the PHS typified by portable telephones, various apparatuses have been remarkably reduced in size. Of the parts constituting the apparatuses, a dielectric filter in which a coaxial dielectric resonator is combined is comparatively large in size and there is a strong demand that the dielectric filter be reduced in size. To meet the demand, as disclosed in Japanese Laid-open Patent Application H3-254513, a laminated filter has come to be used which is manufactured by laminating dielectric green sheets having conductive paste printed thereon and sintering the lamination. Although advantageous in size reduction, the laminated filter is not satisfactory in characteristics such as insertion loss and out-of-band attenuation. Because of the disadvantage in characteristics of the laminated filter, a dielectric filter using a coaxial resonator is widely used although the size is greater.
FIG. 1 shows the structure of a coaxial dielectric resonator. To manufacture the coaxial dielectric resonator, dielectric powder is filled into a metal cast and pressurized and the obtained compact is sintered, and then, a conductor is formed on a necessary portion on the surface. Here, reference numeral 1 represents a dielectric having its surface metallized and reference numeral 2 represents a through hole.
However, by such dry pressing of powder, pressure cannot be uniformly applied, so that there is a limit in size reduction. Therefore, conventional coaxial dielectric resonators are approximately 1.8 mm square at the minimum.
As described above, although there is a strong demand that the coaxial dielectric resonator be reduced in size, a very small size resonator, for example, a resonator 1.6 mm square or smaller cannot be manufactured according to the conventional manufacturing method. Such a very small size coaxial resonator is strongly demanded because if such a resonator is realized, a dielectric filter will be obtained which is as small in size as the laminated filter but has more excellent characteristics.